1. Field of the Invention
The present invention relates to a DC motor driven power steering system for a motor vehicle for assisting a driver in steering the motor vehicle with an assist torque generated by a DC motor. More specifically, the present invention is concerned with a DC motor driven power steering system which is equipped with a facility for detecting the state of operation or rotation of the DC motor with high reliability, wherein information of the rotation state is utilized for controlling generation of the assist torque.
2. Description of Related Art
In the power steering system for automobiles or the like motor vehicles in which a DC motor is employed for generating the assist torque, it is required to detect the operation or rotation state of the DC motor through a feedback loop in order to generate the optimal steering assist torque while allowing detection of occurrence of abnormality in the power steering system.
Under the circumstances, in the DC motor driven power steering system known heretofore, a sensor means such as a rotary encoder, a tachometer generator or the like is employed for detecting an angular position and/or rotation speed of the DC motor as the information to be utilized for controlling the angular position as well as speed and acceleration of the DC motor to thereby control ultimately the steering assist torque.
However, the sensor means for detecting the motor information such as the rotation number of the DC motor and others as mentioned above is very expensive, which is of course disadvantageous from the economical viewpoint. For this reason, there have heretofore been proposed a variety of approaches which are designed for obtaining the rotation information of the DC motor by arithmetic estimation based on the outputs of the other sensors which are installed for the other purposes without resorting to the use of dedicated detecting means such as the rotary encoder or the like.
By way of example, there is disclosed in Japanese Unexamined Patent Application Publication No. 8190/1992 (JP-A-4-8190) a system in which a motor voltage applied across a DC motor and a motor current flowing therethrough are detected for estimating the rotation speed or number (rpm) of the DC motor on the basis of the detected motor voltage and the motor current. In another known system also disclosed in the above-mentioned publication, the motor rotation number is estimated on the basis of a motor current command value for commanding the current to flow through a DC motor and a motor current actually flowing through the motor. In any case, the motor rotation number estimated in this way is utilized in the steering control.
However, the motor rotation number estimating techniques mentioned above suffers a serious problem that the estimated value of the motor rotation number is often accompanied with remarkably large error in dependence on the motor driving method as adopted because the motor rotation number is estimated (or arithmetically determined, to say in another way) on the basis of the motor voltage and the motor current of the DC motor while taking into account the internal parameters thereof such as armature resistance, self-inductance, torque constant, viscosity resistance coefficient of the motor shaft, inertia of the rotor and others.
For better understanding of the background of the present invention, description will first be made in some detail of a DC motor driven power steering system known heretofore by reference to FIGS. 8 to 11.
FIG. 8 is a schematic diagram showing in general a structure of a conventional DC motor driven power steering control apparatus for a motor vehicle. As can be seen in the figure, a steering wheel 1 is operatively coupled to a steering shaft assembly which is composed of first to fourth steering shafts 2a to 2d so that rotational motion or torque of the steering wheel 1 is transmitted to the steering shafts.
A torque sensor 3 is provided in association with the steering wheel 1 for detecting a torque applied by a driver or operator to the steering wheel 1 for thereby outputting an electric signal T1 indicating the steering torque. More specifically, the steering wheel 1 and the torque sensor 3 are interlinked by means of the first steering shaft 2a. Additionally, connected operatively to the torque sensor 3 is the second steering shaft 2b at one end thereof, while a first gear 4 is mounted on the second steering shaft 2b at the other end thereof and meshes with a second gear 5, wherein the first and second gears 4 and 5 cooperate to constitute a reduction gear train. Further, the first gear 4 is connected to a first universal joint 6a by way of the third steering shaft 2c. The first universal joint 6a in turn is connected to a second universal joint 6b via the fourth steering shaft 2d. A pinion 7 is mounted on the second universal joint 6b and adapted to mesh with a threaded portion 8a of a rack 8. Mounted on the rack 8 at both ends thereof are first and second ball joints 9a and 9b, respectively, wherein tie rods 10a and 10b are coupled to both ends of the rack 8 via the first and second ball joints 9a and 9b, respectively. In addition, a DC motor 11 is operatively coupled to the second gear 5 for generating an assist torque to be applied to the power steering system under the control of a control apparatus 12 which is designed to control the steering operation in accordance with the electric signal T1 delivered from the torque sensor 3 to thereby assist the driver in steering the motor vehicle. The control apparatus 12 is supplied with an electric power from an onboard battery 13.
FIG. 9 is a block diagram showing generally a functional configuration of the control apparatus 12 shown in FIG. 8, i.e., a conventional DC motor driven power steering control apparatus. In FIG. 9, reference numerals 3, 11 to 13 denote same components as those designated by like numerals in FIG. 8. The control apparatus 12 for the DC motor driven power steering system is composed of elements 20 to 24 and 29 to 33, as described below.
Referring to FIG. 9, the control apparatus 12 includes a central processing unit (hereinafter referred to as the CPU in abbreviation) 20 which is programmed to perform various arithmetic processings involved in the control of the motor driven power steering operation. A power supply circuit 21 is connected to the battery 13 for supplying electric energy to various components of the control apparatus 12, although the power supply circuit 21 is shown to supply the electric power to the CPU 20 for simplification of illustration. An input interface circuit 22 is provided for fetching and conditioning the electric signal T1 outputted from the torque sensor 3. More specifically, the electric signal T1 indicative of a steering torque applied to the steering wheel 1 by the driver is inputted to the CPU 20 via the input interface circuit 22 to be processed thereby in the manner which will be elucidated later on, whereby a motor driving signal DM is outputted from the control apparatus 12. A motor drive circuit 23 is provided for generating PWM (Pulse-Width Modulated) control signals PC1 to PC4 on the basis of the motor driving signal DM. Further, a motor current detection circuit 24 is provided to detect a motor current IM flowing through the DC motor 11 for generating a motor current signal IM which is inputted to the CPU 20. Four switching elements such as electric field effect transistors (hereinafter referred to as the FET in abbreviation) 29 to 32 cooperate with the DC motor 11 to constitute an H-bridge commutation circuit BR. One pair of the FETs 29 and 32 as well as the other pair of the FETs 30 and 31 is adapted to be controlled or turned on and off by the PWM control signals PC1 to PC4 so that the DC motor 11 is caused to rotate reversibly in either the forward or backward (reverse) direction for generating the steering assist torque. A source voltage detection circuit 33 serves to detect a power supply voltage supplied to the bridge commutation circuit BR from the battery 13, wherein the output signal VB of the circuit 33 indicating the power supply voltage VB as detected is also inputted to the CPU 20. Finally, there is provided a means for detecting a motor voltage VM which is applied to the DC motor 11, although this detecting means is omitted from illustration.
Next, operation of the conventional DC motor driven power steering system of the structure described above will be reviewed briefly.
The CPU 20 generates the motor driving signal DM on the basis of the electric signal T1 outputted from the torque sensor 3 and at the same time estimates the rotation number (rpm) of the DC motor 11 on the basis of the motor current IM supplied from the motor current detection circuit 24 and the motor voltage VM supplied from the motor voltage detecting means (not shown). On the other hand, the motor drive circuit 23 generates the PWM control signals PC1 to PC4 on the basis of the motor driving signal DM for thereby driving the DC motor 11 via the FETs 29, . . . , 32 of the bridge commutation circuit BR. The torque thus generated by the DC motor 11 is transmitted to the steering shafts 2b and 2a via the gear train composed of the second gear 5 and the first gear 4, as a result of which an assist torque of appropriate magnitude and direction is applied to the steering wheel 1.
Next, description will turn to operation of the CPU 20 for estimating the rotation speed or number (rpm) of the DC motor 11 by reference to FIGS. 10 and 11 which graphically illustrate motor voltage (VM)-versus-motor current (IM) characteristics of the DC motor 11 with temperature thereof being used as a parameter, wherein FIG. 10 shows the characteristic in the case where one (e.g., 29) of each pair (e.g. 29 and 32) of the FETs 29 to 32 constituting the bridge commutation circuit BR is turned on and off by the PWM control signal while the other (e.g. 32) is held constantly in the closed or ON-state, whereas FIG. 11 shows the characteristic in the case where both of the paired FETs (e.g. 29 and 32 or 30 and 31) are turned on and off by the PWM control signal simultaneously or independently from each other.
More specifically, the voltage-versus-current characteristics illustrated in FIG. 10 are obtained when the DC motor 11 is driven by turning on/off the FET 29 with the FET 32 being maintained in the on-state while the FETs 30 and 31 are held in the off-state by the PWM control signals PC1 to PC4 outputted from the motor drive circuit 23. On the other hand, the voltage-versus-current characteristics shown in FIG. 11 are obtained by driving the DC motor 11 by switching both the FETs 29 and 32 with the PWM control signal while holding the FETs 30 and 31 in the off-state.
In both of FIGS. 10 and 11, the motor current IM flowing through the DC motor 11 is taken along the abscissa with the motor voltage VM, i.e., the voltage applied across the DC motor 11 in the non-rotating state thereof being taken along the ordinate, wherein a curve A represents the characteristic when the motor temperature is at a room temperature, a curve B represents the same at a high motor temperature, and a curve C represents the same when the motor temperature is low. Further, in FIGS. 10 and 11 reference characters IF1 and IF2 designate, respectively, lower limit values of the motor current IM for the ranges in which the characteristic curves A to C exhibit linearity.
As can be seen in FIGS. 10 and 11, the relation between the motor voltage VM and the motor current IM changes as a function of the temperature of the motor. In other words, when the DC motor 11 is at a room temperature, the relation mentioned above is such as represented by the characteristic curve A. At a higher temperature of the motor 11, the relation is such as represented by the characteristic curve B, while at a lower temperature of the DC motor 11, the motor voltage VM thereof changes as a function of the motor current IM as indicated by the characteristic curve C. Furthermore, when the motor current IM is smaller than the lower limit value IF1 or IF2, all the characteristic curves A to C assume nonlinear forms, respectively.
Referring to FIG. 10, let's assume that the DC motor 11 is in the non-rotating state when the motor current IM is equal to a value ID. In that case, the motor voltage VM assumes a value which falls within a range given by the expression VMD1.gtoreq.VM.gtoreq.VMD2. Further, in the case where the DC motor 11 is in the non-rotating state when the motor current IM assumes a value IQ in the driving mode illustrated in FIG. 11, the motor voltage VM assumes a value within a range given by the expression VMQ1.gtoreq.VM.gtoreq.VMQ2.
Furthermore, comparison of the characteristic curves illustrated in FIG. 10 with those of FIG. 11 shows that the characteristic curves A to C rises up rather gently at a relatively low rate of the motor voltage VM when one of each pair of the FETs is controlled by the PWM control signal (FIG. 10), while the characteristic curves A to C rises up steeply at a relatively high rate of the motor voltage VM when both FETs of one pair are controlled by the PWM signal (FIG. 11).
It is further to be mentioned that when the DC motor 11 is rotating in a desired direction under a load, there will be detected the motor voltage VM which is lower than that given by the characteristic curve A, B or C while when the DC motor 11 is rotating in the regenerative mode (i.e., in the direction in which electric power is generated by the DC motor 11), the motor voltage VM which is higher than that given by the characteristic curve A, B or C will be detected.
As will now be appreciated from the foregoing discussion, the rotation number (rpm) of the DC motor 11 can arithmetically be determined or estimated on the basis of a difference between the motor voltage VM detected at a given motor current IM (=ID) and the motor voltage (VMD1 to VMD2) or motor voltage (VMQ1 to VMQ2) located on the relevant characteristic curve.
However, the characteristic between the motor current IM and the motor voltage VM becomes different as the motor temperature changes regardless of the driving scheme as adopted, as indicated by the characteristic curves A to C in FIGS. 10 and 11. Besides, the characteristic curves A to C will change under the influence of heat generation internally of the DC motor 11.
Additionally, the motor voltage-versus-motor current characteristics differ remarkably from one another in respect to the rise-up voltage, slope thereof and the lower limit values (IF1, IF2) of the linear ranges. Of course, the characteristic will vary significantly in dependence on the motor driving schemes as adopted (compare FIG. 10 with FIG. 11).
By way of example, in the case of the characteristics illustrated in FIG. 10, although a relatively large margin can be ensured for the detection of a high motor voltage VM, the margin for the detection of a low motor voltage becomes small. By contrast, in the case of the characteristics illustrated in FIG. 11, a large margin is available for the detection of the low motor voltage VM whereas the margin for the detection of high motor voltage VM is narrowed.
As will now be apparent from the above analysis, estimation of the rotation number or speed (rpm) of the DC motor 11 on the basis of only the motor voltage VM, the motor current IM and the internal parameters thereof will be accompanied with significant error due to the various influential factors such as those mentioned above. In order to decrease the error involved in the estimation, it is required to change the method for estimating the rotation number of the DC motor 11 in dependence on the schemes or modes for driving the DC motor 11. However, there has been proposed no DC motor driven power steering system which incorporates the means for changing the estimation methods as mentioned above.
Finally, it should also be mentioned that in the DC motor driven power steering system in which the motor driving scheme where the one of each pair of the FETs is held in the conducting or ON-state with the other being switched or controlled by the PWM control signal (FIG. 10) can be changed over with the scheme where both FETs of each pair are switched on and off by the PWM control signal simultaneously or independently from each other (FIG. 11), error involved in the estimation of the rotation number of the DC motor 11 will become different every time the driving schemes mentioned above are changed over.
In the DC motor driven power steering system known heretofore, estimation of the rotation number or speed of the DC driving motor on the basis of the motor voltage VM, the motor current IM and the internal parameters without resorting to use of any proper rotation number detecting means which are generally expensive results in remarkably large error, as described above, as a result of which realization of optimal steering control is difficult or practically impossible, giving rise to a problem.
Furthermore, in the conventional DC motor driven power steering system in which the scheme for driving the DC driving motor by holding one of each pair of the FETs in the ON-state while turning on/off the other FET by the PWM control signal (FIG. 10) is changed over with the driving scheme in which both the FETs of each pair are turned on/off by the PWM control signal simultaneously or independently (FIG. 11), error involved in the estimated value of the motor rotation number becomes different significantly every time the aforementioned motor driving schemes are changed over, incurring error in the estimated or detected motor rotation number.